1. Field of the Invention
The present invention relates generally to membrane contactors for fluid separation and processing, and more particularly to shell-less hollow fiber membrane contactors and methods of fluid processing utilizing shell-less hollow fiber membrane contactors.
2. Description of Related Art
Membrane contactors provide a means of accomplishing gas/liquid, and liquid/liquid (which can encompass liquid/dissolved solid) separations. Membrane contactors typically are used to bring two immiscible fluid phases-for example, a first liquid and a second liquid, or a gas and a liquid-into contact with one another to effect separation and/or transfer of one or more components from one fluid to the other.
A membrane contactor is a modular device, commonly including a bundle of microporous hollow fibers. Membrane contactors typically include a rigid shell or housing enclosing the fiber bundle. The shell typically is provided with four fluid ports: an inlet for introducing the first fluid, an outlet for discharging the first fluid, an inlet for introducing the second fluid, and an outlet for discharging the second fluid. The hollow fibers are potted on both ends, within the housing, to form polymeric tube sheets with the fiber bores opening on each end into common first and second end cap portions of the shell. The first end cap contains the inlet for the first fluid, which is designated the "tube-side" or "lumen-side" fluid because it is the fluid that passes through the internal lumens of the fibers. The second end cap contains the outlet for discharging the lumen-side fluid. The second fluid, designated the "shell-side" fluid, typically enters and exits the housing through inlet and outlet ports arranged between the tube sheets, whereby the shell-side fluid contacts the external surfaces of the fibers. The shell-side fluid flows through the interstices between fibers of the fiber bundle, and may be directed to flow parallel or perpendicular to the fiber length. U.S. Pat. No. 5,352,361 to Prasad, et al., incorporated by reference herein in its entirety, may assist in a background understanding of fluid contact across hollow fiber membranes within a shell.
Because the tube sheets separate the lumen-side fluid from the shell-side fluid, the lumen-side fluid does not mix with the shell-side fluid, and the only transfer between the lumen-side fluid and the shell-side fluid occurs through the walls of the fibers. The fine pores in the fiber wall are normally filled with a stationary layer of one of the two fluids, the other fluid being excluded from the pores due to surface tension and/or pressure differential effects. Mass transfer and separation are caused by diffusion, which is driven by the difference in concentration of the transferring species between the two phases. Typically, no convective or bulk flow occurs across the membrane.
In the case of gas/liquid separations, membrane contactors are typically fabricated with hydrophobic hollow fiber microporous membranes. Since the membranes are hydrophobic and have very small pores, liquid will not easily pass through the pores. The membranes act as an inert support that brings the liquid and gas phases into direct contact, without dispersion. The mass transfer between the two phases is governed by the difference in partial pressure of the gas species being transferred.
For liquid systems, the liquid/liquid interface at each pore is typically immobilized by the appropriate selection of membrane and liquid phase pressures. In this case, the membrane also acts as an inert support to facilitate direct contacting of two immiscible phases without mixing.
Membrane contactors can be utilized for a variety of applications, including the separation of a component from a fluid or transferring a component of one fluid to another. For example, a membrane contactor can be used in removal of contaminants from an effluent stream. In many industrial processes, a contaminated effluent stream is generated as a by-product. In view of environmental concerns, and/or efforts to improve process efficiency, it is often desirable to remove one or more contaminants from the effluent stream so that the contaminant does not pollute the environment, or so that it may be recycled. Existing industrial processes frequently must be upgraded to reduce environmental emissions and/or increase efficiency. Therefore, a need often arises for a process and system that can be economically retrofit to an existing plant to reduce emissions.
Several factors are important in the design of membrane contactors, including separation characteristics, cost, pressure drop, weight, and efficiency. The pressure drop across a contactor should be low to reduce the need for more expensive high pressure equipment. Low pressure drop is of particular importance in retrofit projects where a membrane contactor is to be added at the discharge point of an effluent process stream, as the process pressure at this point is typically at or near atmospheric pressure. High efficiency of mass transfer is desirable for reducing the size of the contactor. Low weight is desirable for decreasing installation and maintenance costs, and is of particular importance in offshore applications. Existing membrane contactors have been found less than fully satisfactory in meeting these goals. For example, the shell portion of typical membrane contactors adds considerably to their weight and expense. Shell-type contactors also typically must operate at elevated pressures. Accordingly, a need exists for a membrane contactor having improved characteristics over known membrane contactors.
It is to the provision of a microporous hollow fiber membrane device and method meeting these and other needs that the present invention is primarily directed.